NASA's STEREO Studies Extreme Space Weather

On July 22, 2012, a massive cloud of solar material erupted off the sun's right side, zooming out into space and passing one of NASA's twin Solar Terrestrial Relations Observatory, or STEREO, spacecraft along the way. Scientists clocked this giant cloud, known as a coronal mass ejection, or CME, as traveling over 1,800 miles per second as it left the sun.

Conversations began to buzz and the emails to fly: this was the fastest CME ever observed by STEREO, which since its launch in 2006 has helped make CME speed measurements much more precise. Measuring a CME at this speed, traveling in a direction safely away from Earth, represented a fantastic opportunity for researchers studying the sun's effects. Now, a paper in Nature Communications, published on March 18, 2014, describes how a combination of events worked together to create these incredible speeds.

"The authors believe this extreme event was due to the interaction of two CMEs separated by only 10 to 15 minutes," said Joe Gurman, project scientist for STEREO at NASA's Goddard Space Flight Center in Greenbelt, Md. "Plus the CMEs traveled through a region of space that had been cleared out by another CME four days earlier."

The researchers describe the July 2012 event as a perfect storm, referring to the phrase originally coined for the October 1991 Atlantic Ocean storm to describe an event where a rare combination of circumstances can drastically aggravate a situation.

Such work helps scientists understand how extreme solar events form and what their effects might be if aimed toward Earth. At Earth, the harshest space weather can have strong effects on the magnetic system surrounding the planet, which in turn can affect satellites and interrupt GPS and radio communications. At its worst, rapidly changing magnetic field lines around Earth can induce electric surges in the power utility grids on the ground. One of the best ways to protect against such problems, and perhaps learn to predict the onset of one of these storms, is to make computer models matching the observations of past events.

In the case of the July 2012 event, three spacecraft offered data on the CMEs: the two STEREO spacecraft and the joint European Space Agency/NASA Solar and Heliospheric Observatory, or SOHO. SOHO lies between Earth and the sun, while the two STEREO spacecraft have orbits that for most of their journey give views of the sun that cannot be had from Earth. Each spacecraft observed the CMEs from a different angle, and together they could help map out a three-dimensional image of what happened.

The authors suggest it was the successive, one-two punch of the CMEs that was the key to the high speeds of the event – speeds that would lead to circling Earth five times in one minute. A CME from four days earlier had an impact too. First, it swept aside particles in the way, making it all the easier for the next CMEs to travel. Second, it altered the normal spiral of the magnetic fields around the sun to a straighter pattern above the region that was the source for these CMEs, thus allowing for freer movement.

"A key finding is that it’s not just the initial conditions on the sun that can produce an extreme space weather storm," said Gurman. "The interactions between successive coronal mass ejections farther out in interplanetary space need to be considered as well."

The researchers found that state-of-the-art models that didn't take the effects of successive CMEs into consideration failed to correctly simulate the July 2012 event. Such information will be incorporated into the models being tested by space weather forecasters. This should lead to better predictions of the worst storms and better protection of Earth and our technology in space.